The invention relates to systems for injecting fuel into the combustion chambers of an internal combustion engine, and in particular a spark-ignition engine; the invention relates particularly to apparatus for estimating the air/fuel ratio admitted into the combustion chambers usable in such systems.
In particular, apparatus is known for estimating the richness of the mixture admitted into each of the n combustion chambers (where n is an integer greater than 1 and generally equal to 4, 6, or 8) of an engine having injectors for injection into the cylinders, the apparatus comprising:
a sensor providing an output signal which varies substantially linearly with richness, the sensor being placed at a junction point between the exhausts from the n chambers; and
computer means for:
storing a model of the behavior of the exhaust at the junction point based on the assumption that the richness at the junction point, i.e. the air/fuel ratio, is a weighted sum of the contributions of the exhausts from the individual chambers, the weighting coefficients decreasing with increasing age of combustion in the chamber, and
after each pass through top dead center, estimating the air/fuel ratio from the measured values and of the model.
Such apparatus is described, for example, in U.S. Pat. No. 5,548,514 or in document EP-A-0 719 922, to which reference can be made.
Such apparatus is suitable for use in particular in an injection system of the kind shown diagrammatically in FIG. 1. The diagram shows an engine 10 having n=4 combustion chambers, each provided with a respective injector 12. The air admitted through a filter 14 passes through a butterfly valve 16 prior to reaching an admission manifold 18. The exhaust gases leave the chambers via individual tubes which are connected together at a junction point leading to an exhaust manifold 20.
The quantities of fuel delivered to each cylinder at injection instants are determined by a computer 21 on the basis of operating parameters which can comprise, in particular:
the angular position of the butterfly valve 16 as measured by a sensor 22;
the pressure in the admission manifold, as measured by a sensor 24;
the temperature xcex8 of the cooling water and/or of the exhaust gases; and
the output signal from a sensor 26 for measuring richness, which sensor is located at the junction point.
The injection instants are fixed to be in advance relative to passes through top dead center in each combustion chamber, by using a synchronizing signal supplied by a sensor 28 facing the flywheel 30 of the engine 10.
A simple model for representing the richness as measured at the junction point consists in associating the measurement performed by the sensor 26 at a plurality of successive passes of the combustion chambers through top dead center, with respective weighting coefficients that are a function solely of the age of the pass in the operating cycle of the engine. The input to the model is the richness of the mixture admitted to the combustion chamber that has just passed through top dead center (the current cylinder). The puffs of exhaust towards the junction point are combined with one another to represent the gas mixture.
There also exists dispersion between the characteristics of the injectors, such that injection of given determined duration does not correspond to the same quantity of fuel being injected into each of the various chambers.
In the case of four combustion chambers, for example, the sensor is associated with a vector of coefficients Ci where i={1, 2, 3, 4}, with C4 corresponding to the current cylinder and with the other, smaller coefficients corresponding to the other cylinders in reverse order of ignition.
That solution is not totally satisfactory because exhaust pipework is generally not symmetrical.
The present invention particularly intends to provide estimation apparatus that satisfies practical requirements better than previously-known apparatuses because it greatly reduces the effects of asymmetries, and specifically, in the event of asymmetry, the invention improves the correction for dispersion in the characteristics of the injectors.
To this end, the invention provides, particularly, apparatus in which the behavior model includes a submodel specific to each combustion chamber and comprising, for each chamber of order i, a Kalman filter having an mxc3x97n matrix of coefficients Cij and a matrix of specific gains Kij, where i is equal to {1, . . . , n} and corresponds to the number of the chamber, and where j lies in the range 1 to m and corresponds to the number of the weighting coefficient. In other words, the invention proposes a different model for each chamber i, as defined by a set (j) of m coefficients, where m may be equal to n.
Such apparatus which makes it possible to avoid the effect of the exhaust being asymmetrical, also has the advantage of greatly reducing the effect of dispersions in the characteristics of the injectors, and consequently makes it possible to use injectors that have been machined with lower precision.
The model can be represented by one or more matrices (Cij)xcex, each corresponding to an operating zone xcex of the engine as determined by one or more parameters selected from load range, exhaust gas temperature, cooling water temperature, engine speed, and pressure in the admission manifold.
Which matrix is selected can also depend on the set richness given by the computer, and it can depend on the operating conditions of the engine and on constraints concerning pollution or drivability.